1. Field of the Invention
This invention relates to an improved switching circuit for redundant, d.c. power supplies, and, more particularly, to a circuit for efficiently coupling and for dynamically decoupling the power sources to a load.
2. Description of the Prior Art
Uninterruptible power supplies commonly have two, redundant, d.c. power sources connected by relays or solid state switches to a single load. If one of the power supplies fails, the load can be carried by the other power supply. In many applications where redundant power supplies are used, it is desirable that the circuit be capable of dynamically disconnecting the load from the power supply; to save power in certain applications and in addition to isolate a system fault, such as a short circuit in the load or in the power supply.
Prior art redundant power supply circuits commonly use a diode connected in series between each power supply, its associated switch, and the load. These diodes serve to isolate faults and prevent them from propagating between the power busses. However, because of the fixed voltage drop across the diode in its forward conducting direction, these diodes reduce efficiency, particularly in low voltage applications (e.g. a 0.5 volt drop across the diode in 5 volt supply system).
Proposals have been made in the prior art to use MOSFET transistors as the switch in redundant power supply applications. Particularly, to connect the MOSFET switch "backwards" between the power source and the load. For example, with a P channel MOSFET, the drain is connected to the positive terminal of the d.c. power supply and the source is connected to the load (i.e. the reverse of the typical circuit connection for a MOSFET). In this configuration, the integral diode of the MOSFET is oriented so that it will not conduct current from the load to the power supply when the MOSFET is turned off. However, in this configuration, the integral diode does provide a conduction path from the power supply to the load with the MOSFET off so that the MOSFET switch does not isolate load faults from the power supply.